Method and means for identifying aircraft



Feb. 6, 1951 D. J. BARcHcK Erm.

METHOD AND MEANS FoR IDENTIFYING AIRCRAFT 15 Sheets-Sheet 1 Filed July 29, 1943 INVENTORS.

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Feb. 6, 1951 DQ J. BARcHoK ETAL 2,540,087

METHOD AND MEANS OR IDENTIFYING AIRCRAFT Filed July 29, 194s 13 sheets-sheet 2 FIG. 2. 32

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INVENToRs. DONALD J. BARcHoK & IRVI NG STOKES.

Feb. 6, 1951 D. J. BARcHoK ETAL METHOD AND MEANS FOR IDENTIF'YING AIRCRAFT 13 Sheets-Sheet 5 Filed July 29, 1943 Feb. 6, 1951v D. J. BARcHoK ETAL METHOD AND MEANS FOR IDENTIFYING AIRCRAFT Filed July 29,` 1943 13 Sheets-Sheet 4 INVENTORS. DONALD J.BARCHOK 8. IRVING STOKES.

BY ww Feb. 6, 1951 D. .1. BARcHoK ETAL METHOD AND MEANS Foa IDENTIFYING AIRCRAFT 13 Shee's-Sheet 5 Filed July 29, 1943 K o m Sm FzoNEoz All-T MR Hm .u E. MM m53@ W. J. .zoNEoIAIII- W. w #do o... ANH o D T mw IP. W ...52.5 m... .bl vzzjmza o E Nm o 2. T motuzuu W I I- :Bbz A m NN n mm 2 2 b wh .M t S tr; Y F

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iff/wey Feb. 6, 1951 D. J. BARcl-lk ETAI.

METHOD AND MEANS FoR IDENTIFYING AIRCRAFT 1s sheets-sheet e Filed July 29, 1943 omhzoo .lle

INVENTORS. DONALDJBARCHOK Feb. 6, 1951 D. J. BARcHoK ETAL METHOD AND MEANS FOR 1DENT1FY1NGA1RCRAFT 13 Sheets-Sheet Filed July 29, 1943 M M Alp-l INVENTORS. DONALDJ. BA R CHO K 8.

IRVI NG STOKES.

Feb. 6, 1951 D. J. BARcHoK ETAL 2,540,087

METHOD AND MEANS FoR IDENTIFYING AIRCRAFT Filed July 29, 1943 1.3 Sheets-Sheet 8 FIG. IO.

INVENTORS. DONALD J. BARCHO K eb. 6, 1951 D. J. BARcHoK ETAL. 2,540,087

mamon AND MEANS FOR IDENTIFYING AIRCRAFT Filed July 29, 1943 13 Sheets-Sheet 9 Fl G. |O. CONT'D) l INVENTORS'. DONALD JBARCHOK IRVING A s'roKEs.

. J. BCHOK ET AL METHOD AND MEANS FOR IDENTIFYING AIRCRAFT 13 Sheets-Sheet 10 Filed July 29,` 1945 FIG. li.

INVENTORS DONALD J.BARCHOK B. IRVING STOKES.

Feb 6, 1951 D. J. BARcHoK ETAL 2540,?

METHOD AND MEANS Fox IDENTIFYING AIRCRAFT Filed July 29, 1943 13 Sheets-Sheet l1 FIG. ll. coN1"o lllll l' INVENTORS. DONALD .LBARCHO K ,'M. @/e IWW/ny.

Feb. 6, i951 D. J. BARcHoK ETAL 295409087 mamon AND MEANS Fox IDENTIFYING AIRCRAFT Filed July 29, 1945 13 Sheets-Sheet l2 FIG. I2.

INVENTORS' DONALDJBA RCHO K & IRVING STOKES.

BY MLWCLM arrry Feb. 6, 1951 D.`J. BARcHoK ETAL 2,540,087

METHOD AND MEANS FOR IDENTIFYING AIRCRAFT Filed July 29, 194s 1s sheets-sheet 1s FIG. l2. ccoNTm INVENTORS. DONALD .LBARCHO K & IRVI NG STOKES.

Patented Feb. 6, 1951 y UNITED STAT-ES METHOD AND MEANS FOR IDENTIFYING- AIRCRAFT nomia J. nmhok, Beim, and Irving swim.

Neptune, N'. J.

Application July 29, 19113, Serial'No. 496,852

(Granted under the act of March 3, 1883. as

amended April 30, i928; 370 0. G. '157) 1'1 claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to us of any royalty thereon.

Our present invention relates to a method and means for interrogating aircraft from the ground whereby distinction may be made between those which are friendly and those which are hostile. and while not limited thereto, it is admirably adapted for use in cooperation with a pulse-echo system for aircraft detection.

In one form of such a system, pulses of high frequency energy are projected into space at an audio frequency rate and are made to scan through 360 of azimuth. Should the energy so projected encounter an object in space such as an aircraft, a portion thereof is reflected and the reflection or echo is received back at the detecting station. A portion of both the original transmission and the echo are applied to an indicating device such as an oscilloscope provided with a horizontal sweep, the original transmission appearing, in the form of a vertical deection, at the commencement of thesweep and the reflected energy or echo appearing, also in the form of a vertical deflection. somewhere along the oscilloscope base line, depending upon the distance between the detecting station and the object causing the echo. There is generated in synchronism with the pulse transmission, and superimposed upon the oscilloscope sweep, a negative-going marker notch the phase of which may be varied with respect to the pulse transmission so that the notch can be moved along the oscilloscope base line. The device for shifting the phase of the marker notch voltage is calibrated in terms Lof distance, so that when the notch is moved along the oscilloscope base line into alignment with any selected echo appearing on such base line, the operator is immediately informed of the range of the object causing that particular echo.

In addition, the reflected echoes are applied' to another oscilloscope, of the PPI (plan position indicator) type, i. e., one provided Withpa radial sweep synchronized with the pulse transmission of the system, said radial sweep being rotatable about its point of origin in synchronism with the azimuthal scanning motion of the system. By means of this oscilloscope the bearing in azimuth of each echo-causing object is indicated by the angular orientation of the radial sweep, while the range of said object is indicated by the position of the echo-indicating signal along said radial sweep.

Heretofore, interrogation equipment has been designed for cooperation with pulse-echo systems for object detection having general characteristics similar to the' one jyist described, but the connection therebetween has necessitated alterations in the detecting equipment which often ich' caused the introduction of inaccuracies in various portions of the detecting system, particularly in the range-determining portion thereof. In addition, the interrogation equipment heretofore employed has been of limited use because of its inability to determine the identity of particular preselected echo-causing objects.

It is therefore an object of the present invention to provide an accurate and fool-proof interrogation system for use in cooperation with an object detection system whereby the possibility of error in identifying detected aircraft is practically eliminated.

It is a further object of the present invention to provide an interrogation system for use in cooperation with an object detection system witbout requiring such alterations as might cause any tuii'avorable interaction between the two sys- It is still another object of the present invention to provide an interrogation system whereby a particular or selected craft of a plurality of craft detected by the detecting system may be challenged to establish the identity thereof.

These and other objects and advantages, which will become obvious to those skilled in the art as the detailed description progresses. are attained in the present invention in the following manner: A portion of the pulse-generating voltage of the detection system is utilized to synchronize the keying of a transmitter in the interrogation system whereby there is periodically radiated into space, in directional alignment with the pulse transmission of the detecting system and in a preselected azimuthal plane, a, challenging signal. The challenging signal triggers a transponder, which is an airborne transmitter carried by friendly aircraft, and the latter automatically transmits a recognition signal upon being so triggered. Since such automatically-triggered transmitters are well known in the art and do not constitute per se any part of our invention, no further description thereof need be given.

The antenna of the interrogation equipment is so designed as to have a dual response pattern and by means of a pattern or lobe-switching mechanism associated with the antenna which, per se, is no part of the present invention and. therefore not shown, the recognition signal receiver responds to signals alternately from the two patterns of the antenna. When the plane of the antenna is normal to a line between the ground station and the aircraft being interrogated, the amplitude of the signals from each antenna pattern will be equal and application of these signals to an oscilloscope, the sweep of u which is synchronized with the above referred to notch-generating volt/age of the detection system range oscilloscope, results in a picture conso tiilng of two adjacent vertical deections of the oscilloscope base unepof equal, height.

Such a picture' corresponds to an "on target" condition and indicates that -the challenged raircraft is friendly. Should the recognition signal oscilloscope of the interrogation system fail to indicate the presence of an object at a timewhen the rangev oscilloscopevof the detecting system displays a reflected echo within the notch in the base line thereof. those on the ground are imme'diately infomed that the craft upon which the equipment is trained is-hostile. Or, should the recognition signal voscilloscope show a picture wherein one of the vertical deflections is of greater magnitude than 'the other. thoseon the ground are immediateLv infomed that there is a friendly craft in the vicinity but it is not in the azimuthal plane to which the antennae are normal andthe craft that'is in the "on-target" position is hostile.

.'In the following specification we describe and in the annexed drawings showa specific embodiment of'. the present invention, but it is to be clearly understood that said embodiment is merely illustrative and is not intended to limit the true spirit and scope` of the invention, as set forth in the appended claims.

In said drawings,

Figure l ris a block diagram of a combined object detection and interrogation system assembled in accordance with the principles of the present invention;

Figure 2 is al typicall picture of the range- Oscilloscope screen ofthe object detection system showing the presence of several targets;

Figure 3 is a typical picture of a recognition signal oscilloscope, a component of the interrogation system of the present invention, in the presence of a friendly aircraft: I

Figure 4 shows one typical appearance of the same oscilloscope in the presence of a hostile aircraft;

Figure 5 is a block diagram indicating the functions of the various components of the interrogation system of the present invention;

vFigure .6 is a schematic diagram of a keyervpulse-generating circuit;

Figure 7 is a schematic diagram of an oscilloscope sweep-generating circuit;

Figure 8 is a schematic rdiagram of an oscilloscope unblanking-voltage-generating circuit;

Figure 9 is a schematic diagram of a video amplifier and an oscilloscope and the manner in which said oscilloscope is connected with the circuits of Figures 6, 7, and 8;

Figures 10-A to 10-N inclusive show the various wave shapes associated with the keyer-pulsegenerating circuit of Figure 6;

Figures 11-A to 1lP inclusive show the various wave shapes associated with the oscilloscope sweep circuit of Figure 7; and p Figures 12A to 12-Jv inclusive showthe various wave shapes associated with the unblankingvoltage-generator of Figure 8.

We no'vv refer more in detail toA the aforesaid illustrativeembodiment of the present invention,

with particular reference to Figure 1 of the drawings'showlng the manner in which an object detection system and an interrogation system may be combined. While a specific form of detection system will be described in this specification it is to be understood that the interroga-Y tion system of the present invention is not limited to use therewith.y Any other appropriate detecting system will serve as well.

In said Figure 1, we show an audio frequency 4 v s oscillator il having a sinewaveoutput. A

of a few microseconds duration.

tion of this' output is fed to a pulse generator il to distortthesameintoshort,sharppulses.each

preferably adapted tovbe rotated through 300 of azimuth, means being provided for stopping the rotation thereof so that the pulse transmission can'be directed in a selected aaimuthal plane.

Another portion of Vthe output of the audio frequency oscillator Il is fed to a sweep generator is where it is distorted into a saw-tooth wave for application to the horizontal deflecting plates of an oscilloscope 2l, by meansof which the range of a vparticular object detectedby the system may be determined and the relative ranges of a plurality of objects may be observed. y

, erator 22.

applied to the vertical deflecting plates ofthe As is well known, the distance separating an object in space such as an aircraftand a detecting station may be determined by measuring the time interval between the transmission of Va pulse of high frequency energy and the reception back at the detecting station of a portion of the original transmission rei'lected by said object, onevhalf of the product of the time delay and 3x 10|,

the.velocity oi' propagation* vof radio waves in meters, giving the distance.. 4Inorder to measure the time delay between the original transmission and the reception of a reflection or Vecho thereof. a third portion of the output of the audio frequency oscillator Il is fed through a suitable phase shifter 2| to a negative-going notch gen- The output of the notchl generator is to center any selected echo therein, the magnitude of the phase shift necessary to so center the echo becomes a measure of the range of the object causing the particular echo. and the phase shifter may therefore be directly calibrated in terms of distance. f

While we have described a keying pulse `generator consisting of a sine wavegenerator and means to distort the sine wave into sharp pulses,

it is to be understood that the pulses may be generated by using a spark gap keyer. -In Athis event, a portion of the pulses so generated may be'used to produce a sinewave voltage which can then be used to generatethe range-dermim ing notch. Y

'Ihe echoes from reflecting objects are preferably picked up by the same antenna as is used for the original transmission and are fed to aV receiver 23 the output of which is applied'to the vertical deflecting plates of the oscilloscope 2l.

As already indicated, these echoes are also applied to the intensity grid of a PPI 0scil10scops 7s in order to'` obtain the location in azimuth of each object detected. This oscilloscope, not entering into the present invention except to inform the operator of the azimuthal bearing of any particular echo-causing object and therefore determining when the scanning operation is to be stopped, has not been shown in the drawings.

The description thus far has been limited to the object detection system and one form of the interrogation systems of the present invention and the manner of combining the same with said detection system is as follows:

A portion of the output of the pulse generator I6 of the detecting system is transformed into a series of negative pulses which are employed for generating the interrogation. keyer pulses. In the interrogation keyer circuit 24 the pulse rate is reduced to avoid jamming the airborne transponder should several installations simultaneously challenge the same friendly aircraft. The keyer pulses thus generated are fed to an interrogation transmitter 25 consisting of a high frequency generator adapted to be inoperative except for the duration of the pulses fed thereto by the keyer. The carrier frequency of the interrogation transmitter is preferably different than that of the detection transmitter to eliminate interference. The output of the transmitter is applied to 'an appropriate antenna array 26 which is preferably mechanically coupled, as indicated at 21, with the detection system anenna I array I8 so as to be rotatable inalignment with said detection system antenna array. However, the interrogation transmitter is put on the air only after a particular echo-causing object has been selected for challenge, the scanning of the detection system has been brought to a halt, and the radiations of both systems are directed in the azimuthal direction of the object so selected.

The interrogation system antenna. array 26 is also connected to a receiver 28 tuned to the transmitter frequency of the airborne unit, and the output of this receiver is applied to the vertical deilecting plates of a recognition signal oscilloscope 29. i

The antenna array 26 is preferably highly directional and is designed to have a dual response pattern, having associated therewith a lobeswitching mechanism 30 whereby the magnitude of the signals picked up thereby and fed to the recognition receiver 28 alternately depend upon the response pattern in operation and the, angle formed by the azimuthal plane of the aircraft whose transmitter has been triggered and the planes of maximum response of each of the antenna patterns.

In order tor display upon the recognition oscilloscope 29 recognition signals from a paricular object, or a negative response therefrom, we provide said oscilloscope with a sweep, applied to the horizontal deilecting plates thereof and generated, by the circuit 3|, in synchronism with the negative-going notch utilized in the detection system for determining the range of an object causing a selected echo.

By this arrangement, when the phase of the notch-generating voltage :Ts adjusted so that the notch in the base line of the oscilloscope 20 is aligned with a, particular echo, a friendly response caribe displayed only from the craft whose range in the azimuthal direction to which the interrogation antenna array is instantaneously responsive, coincides with the phase displacement of the notch, so that positive means is thereby afforded for identifying the craft whose echo has been selected and dropped into said range-oscilloscope notch.

In Figure 2 ofthe drawings, we show the screen 32 of the range oscilloscope 20 of the detecting system with an instantaneous display of the main pulse transmission 33 and three echoes 34, 35, 38 indicating three objects at'different distances in a single azimuthal plane with respect to the detecting station. The object causing the echo 34 has been selected for interrogation and therefore this echo appears within the notch 31. When. as shown in Figure 3 of the drawings, the screen 38 of the recognition oscilloscope 29 of the interrogation system displays, at the commence.

ment of the sweep, two adjacent vertical deflections'39 and 40 of equal height, the equipment is trained upon the object causing the selected object 34 (Figure 2) and said object is therefore friendly. However, as shown in Figure 4 of the drawings, if the deflections 4i and 42 are of unequal magnitude then the object transmitting these signals is not in the on target plane to which the equipment is adjusted and the object causing the echo appearing within the notch of the oscilloscope 20 is hostile. It will also be apparent that should the oscilloscope screen 38 display no recognition signals while an echo appears in the notch on the screen 32 of the rangeoscilloscope, such lack of signals being deemed a negative response, the object causing said echo must also be hostile.

A fuller understanding of the synchronizing or keyer pulse generating section of the interrogation system of the present invention may be had by reference to the block diagram in Figure 5 of the drawings. As there shown, a negativepulse generator 43, synchronized with the pulse generator or keyer I6 of the detecting system (Figure 1), has its output fed to an inverted 44. The ouput of the inverter is applied to a cathode follower stage 45 and the output of this stage is fed to a frequency divider. 46. As previously stated, the usual pulse frequency of a deection system is too high to employ for keying the interrogating or challenging signal of an interrogation system. Should several ground installations be directing their challenging sgnals at the same aircraft simulaneously, the high detection system frequency would jam the airborne unit or transponder and it is to `eliminate this possibility that the pulse frecluency of the interrogation system is made considerably lower than that of the detecting system.

The reduced freouency output is inverted as at 47. am'plied as at 48, and then passed through a cathode follower stage 49 after which it is employed for keying the challenging signal transmitter 25 (Figure 1) of the interrogation system.

Of course, should the pulse rate of the detection system be fairly low to begin with, it may very well be unnecessary to reduce the same prior to keying the interrogation system transmitter.

One form of circuit which may be used for generating the interrogation keyer pulse is shown in Figure 6 of the drawings, this circuit consisting of the following:

lNegative pulses. from the generator 43 are fed to an unbiased vacuum tube 56 the plate output of which is applied to a vacuum tube 5I- connected as a cathode follower stage. 'Ihe cathode output of this stage is fed to the frequency divider 46 which consists of a pair of vacuum tubes 52 and 53. Plate voltageto these tubes is supplied respectively .through retistors .54 and 55.

lower stage. is employed as the synchronizing voltage for the amos? y the latter being ofv lesser value than the former,

resulting in the tube Il beingoprovided with a higher plate voltage. The cathodes of the tubes l2 and Il are grounded through a common resistor Il and the output of the tube I! is coupled pling including a pulsing network comprising a capacitor 02 and resistor el. The inverted loutput of the tube 00 is applied to an amplifying tube M andthe output of the latter is applied to a vacuum tube l5 connected las a cathode fol- The cathode output of the latter interrogation system transmitter 25 (Figure 1).

The mode of operation of the synchronizing or keyer pulse generating section of the interrogation system just described will be understood by reference to the wave shapes lshown in Figures ,l0-A to lil-N inclusive.'

As shown in Figure ,l0-A, the inputto the vacuum tube 50 consists of negative pulses of the same frequency as the keying pulses of the detecting system. The plate output of this tube therefore consists 'of positive pulses, this being shown in Figure -B. Figure 10-C shows the input, also positive pulses, to the vacuum tube 5I and. inasmuch as this tube'is connected as a cathode follower, the cathode output thereof also consists of positive pulses as shown in Figure 10-D. This output is applied to the grid of the vacuum tube 52, the input to this tube being shown in Figure IO-E.k

For the description of the operation of the frequency divider 48 it will be assumed that the vacuum tube 52, is non-conducting, that the capacitor 51 is fully charged, and that the grid l2, a very high positive potential on the grid of v the tube 53. and a very low voltage at the plate of said tube 5I. Upon the application of the first positive pulse shown in Figure Al0 -E to the grid of the tube 52, the bias onfsaid tube, developed by current owingv through the resistor 56 as a result of the conducting condition of the tube 53,' is overcome and said tube starts to conduct. As a result, as shown in Figure 10-F, the plate voltage on said tube drops. Also as a result of the conducting condition of this tube, the capacitor 51 discharges to a certain valve therethrough by wayof the resistors Il and 59 and. as shown in Figurev 10-G, the grid of the tube 5l is driven to a high negative potential, biasing said tube considerably beyond cuto. As shown in Figure 10-H, the plate voltage on the tube 5I therefore rises.

Because of the non-conducting condition of the tube 53,- the current flowing through the resistor 5I is now reduced and therefore, even after the first positive pulse I put to the tube 52 has passed, the latter contin es to conduct independently of the input ther cyto, so that the next few positive pulses on the grid thereof have no effect on the output of the tube 5I and frequency reductionistherebyattained. Assoon'as thelrlt positivepulseonthetube lihaspalaexLsaid severalpulsestothetubellwillpalduring which time the charge on the capacitor l1 gradually rise, as will the potential on of the tube Il. The time required for Il to become conducting againwili be determined portion in the center of the voltage wave shown in Figure 10-6.V When the tube Il against becomes conducting( current again flows throh the resistor 5l biasing the tube 5I to cuto. The result is an increase in the plate voltageof the tube 52 and a sudden surge of current through the capacitor l1 which, in turn. results in a sudden Jump .in the'v drop across the resistors Il and 59 and therefore in the positive voltage on the grid of the tube Il. AAfter the charge 'on the capacitor 51 reaches its maximum value, the rise of the voltage on the grid of the tube Il tapers oil', as does the rising voltage on the plate of the tube 52, as shown toward the right re.

` spectively in Figures 10-G and l0-F. At the instant when the tube 53 again becomes conducting and the voltage on the grid thereof suddenly pumps in a positive direction, the plate voltage on said tube 53 drops in a negative direction al shown at the right in Figure l0-H. It com-Y mences to rise slowly as the capacitor 51 passes its maximum charge and then, when the capacitor suddenly discharges, as previously described upon the application of a positive vpulse to the grid of the tube 52, it again suddenly rises to its maximum positive value. i

'I'he substantially square-wave output of the tube 53, which is of reduced frequency, becomes distorted into negative and positive-going pulses through the functioningof the pul-se-forinlim network 6I comprising the capacitor 52 and resistor 63, the drop in the plate voltage of the tube 53 resulting in a negative pulse and the rise in said voltage resulting in a positive pulse, as shown in Figure 10-1.

These negative and positive pulses` are fed to the tube 80, which, being biased close to cuto. can substantially amplify only the positive pulses, as shown in Figure 10-J. The input to the tube 64, shown in'Figure lil-K, becomes inverted and amplified by said tube, the inverted, amplified voltage being shown in Figure 10-L. This volt. age, as shown in Figure 10-M, is applied to the grid of the vacuum tube 65 connected as a cathode follower, the bias on this tube being such that the negative input thereto is clipped and the output in the cathode circuit thereof consists only of positive pulses, as may be seen in Figure 10-N. By comparing Figures 10-A and lO-N it will be seen that the pulse frequency has been reduced, the final output of reduced frequency being employed for keying the challenging signal generator 25 (Figure l) of the interrogation system.

--We shall now describe the sweep-generating section of the interrogation system oilloscope of the present invention, for a functional understanding of which reference may be had to Figure 5 of the drawings. The output ofthe notch generator 22 of the detecting system which, it

will be recalled, is synchronized with the pulse'- ognition signal oscilloscope when the system is adjusted for operation over different range limits. The width of the pulses which control the sweep generating circuit governs the speed of the sweep.

The pulse output of the relaxation oscillator 61 is fed to an inverter 68 and the output of the latter is applied to a saw-tooth generator 69. A portion of the saw-tooth wave is directly applied to one of the horizontal deecting plates of the oscilloscope 29 (Figure 1) to obtain a horizontal sweep. Another portion of the sweep generator output is fed to an amplier where it is mixed with a square-wave voltage developed in a spread-voltage generator 1| controlled by a switching mechanism 12 synchronized with the antenna lobe-switching mechanism 39 (Figure l). By means of this arrangement there is introduced into the amplifier 1D a square-wave voltage synchronized with the switching of the antenna response pattern, the mixed spread voltage and sweep voltage being applied to the remaining horizontal defiecting plate of the oscilioscope 29. This results in a push-pull sweep and the alternate displacement of the electron beam of the oscilloscope 29 to one side and then the other of its normal center position whereby the oscilloscope sweep is made to consist of two colin'early displaced and partially overlapping base lines. rI'hese base lines are alternately receptive of the output of the recognition receiver 28 (Figure 1), corresponding to the dual response pattern of the antenna array 26 and enabling the double image display tol which reference has heretofore been made.

One form of circuit which may be employed for generating an oscilloscope sweep of the character just described is shown in Figure '7 of the drawings. As there shown, the negative-going output of the notch generator 22 is fed to an unbiased vacuum tube 13 and the amplified and inverted output of this tube is applied to the normally quiescent relaxation oscillator 61, which includes a pair of vacuum tubes 14 and 15. Plate voltage is supplied to these tubes respectively through resistors 16 and 11, the latter being of lesser value than the former so that the voltage applied to the plate of the tube is higher than that applied to the tube 14. The output of the tube 14 is coupled to the input of the tube 15 through the series capacitor 18 and one or the other of a pair of shunt resistors 19 and 89, these resistors being of dilerent values, and selection of one or the other being made through a switch 8|. The cathodes of the tubes 14 and 15 are grounded through a common resistor 82. By this arrangement the width of the output of the relaxation oscillator 61 is made variable, as will hereinafter be more fully described.

The cathode output of the relaxation oscillator 51 is employed, as will later be set forth, for controlling a brightness-control circuit for the oscilloscope 29 (Figure 1); and the plate output of this oscillator is fed to a vacuum tube 88, functioning as an inverter. The output of this tube is directly applied, without any intermediate coupling, to the sweep generator 69. This generator includes a vacuum tube 84 the cathode of which is connected in series with the plate of a tube 85. Bias is applied to the tube 85 such that said tube draws a substantial current. Connected intermediate the plate and-cathode of the tube 84 is a pair of saw-tooth generating capacitors 88 and 81 the selection of either of which may be made through a switch 88, these capacitors c'ooperating with the resistors 19 and 80 of the re'- laxation oscillator 81 to determine the speed oi' the sweep of the oscilloscope 29. The output across the selected capacitor 86 or 81, is applied to one of the horizontal defiecting plates of said oscilloscope, I

A selected portion of this outputis also applied to a vacuum tube 89 which, in cooperation with another vacuum tube 90 comprises the spread-voltage generator 1|. Bias is applied to the grid of the tube 90 through a resistance'network 9|, the instantaneous value of said bias being controlled by the antenna switch 12 which operates, as previously indicated, in synchronism with the lobe-switching mechanism 30 of the an-Y tenna array 28 to periodically alter said bias to conform to a square wave.

The cathodes of the tubes 89 and 98 are grounded through a com'mon resistor 92 and the plate output'of the tube 89, which, as will later be explained, is a saw-tooth voltage superimposed upon a square-wave voltage, is applied to the reraining horizontal deiiecting plate of the recognition oscilloscope 29 to complete the push-pull sweep and affect the colinear baie line displacement to .vi ich reference has already been made.

The mode of operation of the circuit just described will best be understood by reference to Figures l'l-A to 11-P inclusive and Figure '1.v As may be seen from Figure ll-A, negative-going notches from the generator 22 are applied to the tube 13 which inverts and ampliles the same, as shown in Figure ll-B. A voltage of the same shape (Figure 11-C) is applied to the grid of the rst tube 14 of the relaxation oscillator 61, a detailed description of the operation of the latter being as follows: It is to be assumed that, by reason of the low value of the resistor 11 as compared with that of the resistor 16, the tube 1li is conducting, the current thereof, flowing through the cathode resistor 82, biasing the tube 14 to cuto. It isfurther to be assumed that the ca pacitor 18 is fully charged and the switch 8| is in engagement with the resistor 80, which is of lesser value than the resistor 19. As the grid of the tube 14 goes positive, as shown at the left of each pulse ln Figure 11-C, the bias on said tube is overcome and the tube commences to conduct. The conducting state of this tube permits the capactor 18 to discharge therethrough by way of the resistor and the resulting voltage drop across this resistor drives the grid of the tube 15 highly negative, as may be seen at the left of each p-ulse in Figure 11-E. This renders the tube 15 non-conducting, reducing the ilow of current through the resistor 82 and permitting the tube 14 to remain conducting, though less so, even after the pulse applied thereto has passed. Of course, as the tube 14 becomes conducting, its plate output goes negative, as shown at the left of each-pulse in Figure 11D, and, as shown at the left of each pulse in Figure ll-F, the plate s utput of the tube 15 simultaneously goes posiive.

After each pulse applied to the tube 14 passes and the tube becomes less conducting, the ca' pacitor 18 commences to recharge and the result- 1l 'ing initial voltage drop across-the resistor Il urges the grid of the tube 1I positive. At the same time the plate voltage on the tube 14 commences to go positive and the plate voltage on the tube .16 commences to go negative. shown respectively toward the right of each pulse in Figures il-EI ll-D, and ll-F. As soon as the drop across the resistor 6l drives the grid of the tube 16 suiilciently positive to render said tube conducting again. -current will again now through `the resistor 62 and the tube 14 will be biased to cutoff. This will cause a sudden increase in the plate voltage on said tube. as shown at the right of each pulse inV Figure 1l-D, and it will Vcause a sudden surge of c urrent to the capacitor thereof varies in accordance with the square wave input thereto.

Without the application of the square-wave cathode-output of the tube n to the tube u the plate output of the latter is as shown in 4l'lgure 11-L and this output. of opposite polarity to the potential across'the sweep capacitor I6 or 81, is applied to the remaininghoriaontal defiecting plate of the oscilloscope to complete the push-pull sweep thereof. However. as the current drawn by the tube 80 is increased upon the application of the positive portion of the square-wave input thereto. the currentowing through the resistor 92 is increased and the bias on the tube 6l. is therefore also increased. The normal current of vthe tube 6 8 therefore decreases and, inasmuch as thenegative saw-tooth wave shown in Figure l1-K is also applied toV the grid of said tube 86, the combined input to to become conducting again, when the cycle of operations described will be repeated.

If it is desired to increase the width of the outycreasing the time constant of the R. C. couplingbetween the tubes 14 and 16 and maintaining the tube 16 at cutofl.' for a longer period. The dotted lines in Figures, ll-E to ll-Ij inclusive show the pulse widths resulting from the use of the higher lvalue resistor 1I. The pulse input to the tube 63 which, as shown in Figure l'l-G, is the same as the output of the tube 1l, results in negative pulses in the plate circuit'of the tube 63, as shown in Figure ll-H, and the same shape wave, shown in Figure 1l-I, is applied to the grid of the tube 64.

As previously indicated, the tube 6 4. in cooperation with the tube Il and the capacitors 6I or I1, form the sweep generator Il and the operation of this circuit is as follows:

' 'I'lie tube -86 is biased to draw a heavy current and. inasmuch as between pulses to the tube I4 the latter is conducting.,no current can flow to the capacitors 66 or 61. If theA switch 6I of the oscillator 61 is engaged with the resistor 60, corresponding to the fast sweep, the switch 66 is engaged with the capacitor 81, assumed to be of lesser value than Athe capacitor 66. As the grid of the tube 84 goes negative upon the application of the pulse input thereto, said tube is rendered non-conducting and the capacitor l1 becomes charged through the tube 6l, as shown in Figure ll-J. The rate of this charge will obviously depend upon the value of the capacitor. The dotted lines in Figures 11J tov ll-L inclusive show pulse widths resulting from the use of the higher value capacitor 86.

The negative potential across the capacitor is lapplied to one of the horizontal plates of the square-wave voltage shown in Figure ll-M, this voltage being applied to the grid of the tube Sl, and resulting in the cathode output of said tube shown in Figure ll-N.' 'Ihe tube 66 is conducting at all times, although the. cathode output said tube has the appearance of the wave shown in'Figure ll-O. As a' result, th'e completeoutput inthe plate circuit of thetube 69 will have the configuration of the wave shown in Figure .1l-P and it is this combined wave which is applied to the remaining horizontal plate of the oscilloscope 29 to cause the previously referred to colinear displacement of the baseline in synchronism with the switching of the response pattern of the antenna array 26 (Figure i) We shall now describe the brightness-control circuit to which reference was made during the description of the output of the relaxation oscillator 61 of the recognition oscilloscope sweep generating circuit. 1 Y It will be recalled that the pulse frequency of the interrogationsystem is preferably considerably lower than that of the detecting system and it will also" be recalled that`the sweep for the recognition signaly oscilloscope is generated in synchronism with ythe pulse frequency c' the detecting system. Inasmuch as it is desirable, in

the interest of clearer reading, to maintain the sweep trace of the recognition signal oscilloscope below the visibility level except for the short Vperiod corresponding to the time necessary for 61, one of the components of the sweep generator just described',"and this may be accomplished as follows:

A portion of the output of the cathode follower 49 is fed, as shown in Figure 5 of the drawings. to a relaxation oscillator Il which transforms the short sharp keying pulsesinto negative-going pulses of considerably greater widthk separated by comparatively `long positive portions. The outputof the relaxation oscillator llis fed toa mixer stage 64 where it is combined with a portion of the output of the relaxation oscillator 61 of the recognition oscilloscope sweep generating circuit. s The combined recognition keyer puise and sweep generating pulse is fed to a cathode follower stage il and the cathode output of this stage isapplied to the intensitygrid of the recognition signal oscilloscope 2l to ycontrol the brightness thereof, maintaining the sweep hace below the level of visibility except duringthe 13 period of the sweep synchronized with the pulse transmission of the interrogation transmitter 25 (Figure l) One form of circuit which may be used for this purpose is shown in Figure 8 of the drawings and tively through resistors 98 and 99, the latter bet ing of lesser value than the former, resulting in a higher plate voltage on the tube 91 than on the tube 96. The cathodes of these tubes are grounded through a common resistor and the output of the tube 96 is coupled to the input of the tube/91 by means of a series capacitor |0| and shunt resistor |02. The time constant of this RC coupling is such as totransform the short, sharp pulses of the keyer pulse generator into pulses of appreciable width, the width being equal to the time representing the maximum range of the detecting system. The cathode output of the relaxation oscillator 93 is applied to the grid of a tube |03, constituting the first portion of the above referred to mixer stage, the second portion of this mixer stage including the ftube |04 receptive of a portion of the output of the relaxation oscillator 61 of the sweep generaton The platesY of the tubes |03 and |04 are directly connected with each other, as are the cathodes of these tubes. The combined plate output of these tubes is applied to the grid of a tube |05 connected as a cathode follower stage and having its cathode output connected with the intensity grid of the recognition signal oscilloscope 29 (Figure 1).

The operation of this circuit may best be understood by reference to the wave shapes set forth in Figures 12-A to 12-J inclusive. As shown in Figure 12-A, positive pulses, synchronized with the transmission of the challenging signals of the interrogation transmitter 25 (Figure 1), are applied to the rst tube 96 of the relaxation oscillator 93. Assuming that the tube 91 is conducting and, that by reason of the current flowing through the resistor |00, the tube 96 is non-conducting, and that the capacitor I0| is fully charged, the application of each positive oi' the tubo 91, and a gradual increase of the bias on the tube 96y through the resistor |00. An increase in the plate voltage on the tube 96, and an increase in the cathode output of the tube 91 is caused thereby, as shown in Figures l2-B andf tube 96 will correspondingly rise, as shown in Figure l2-B, as will the cathode output of the tube 91, as shown in Figure 12-D. Between the pulses applied to the grid of the tube 96, and following the initial surges of charging current to the capacitor |0|, there will be a gradual decrease in the positiveness of V,the grid ot the tube 91, as shown at the long center portion of Figure 12-0.

The input to the tube |03, shown in.Figure 12-E1,v is inverted by said tube, as shown in' Figure 12-F, and the output' of this tube is mixed, as will now be described, with a portion of the output of the relaxation oscillator 61 of the recognition-signal oscilloscope sweep generating circuit.

pulse to the grid of the tube 96 renders such y tube conducting. This permits the capacitor |0| to discharge through said tube by way of the resistor |02. The large surge of the discharge current through the resistor |02 drives the grid of the tube 91 highly negative, as shown at the left of the pulses in Figure 12-C of the drawing. This reduces the current flowing through said tube and also through the resistor |00, resulting in maintaining the tube 96 in a conducting state even after the rst pulse applied thereto has passed. The conducting state of the tube 96 decreases the plate voltage thereof, as shown at the left of the pulses in Figure 12-B, and the non-conducting state of the tube 91 decreases the cathode output of said tube, as shown at the left of the pulses in Figure 12-D. After the initial s urge of discharge current through the resistor 02, the drop across said resistor decreases and as a result, the grid of the tube 91 commences to go positive again, as shown at the Such output, shown in Figure 12-G, consists of negative pulses separated by relatively-long positive periods, the negative pulses being synchronized with the pulse transmission of the detecting system. I'hese pulses are applied to the grid of the tube |04 and are inverted thereby, as shown in Figure 12-H. The plate output oi the tube |03 and that of the tube |04 are mixed, resulting in an input to the tube |05, consisting of strong positive pulses-separated by groups of relatively weak positive pulses, as shown in Figure l2-I. The tube |05, connected as a cathode follower, is so biased as to pass only the strong positive pulses of the input thereto, as shown in Figure 12-J, and the resulting output is applied to the intensity grid of the recognition oscilloscope to render the trace on the screen of said scope visible only for the duration of each sweep corresponding to the pulse transmission of the interrogation transmitter 25, said intensity grid being maintained at such a negative level during the periods intermediate these particular sweeps. as to maintain the trace invisible.

We shall now describe the i'lnal section of the interrogation system of the present invention, and the manner of connecting the same, together with the previously described sections, with the recognition signal oscilloscope 29. As may be seen from Figures 5 and 9 of the drawings, the output of the recognition receiver 28, which consists of the signals picked up by the antenna array 26 (Figure l), is fed to a video amplifier |01 which includes the vacuum tube |08. The amplified output of the tube |06 is applied to the vertical deecting plates |09 of the cathode ray ktube l I0 of the recognition oscilloscope 29.

The tube ||0 has its cathode connected to a point ||2 on a bleeder resistory 3 disposed across a. source of high voltage having its positive end grounded. The intensity grid H4 of the tube ||0 is connected, through an adjustable arm H5 to the bleeder ||3 at such a point as to make the grid ||4 negative with respect to the cathode The rst anode ||6 of the tube ||0 is also connected through an adiustable arm ||1 to the bleeder ||3, this point of contact being considerably positive with respect 

